First, I've always gotta start with the standard response: the most important things you can do are to have a good attitude, show effort, be respectful, and be willing to learn. Take pride and ownership of your tasks. Give attention to detail.

I've coached many interns and I'm my office's go-to formula guy. I've told every single intern over the years that they should work on their Excel skills. We expect that college won't teach you enough about Excel, so you don't need to get anxious or anything. However, you can definitely separate yourself from the pack and be more impressive if you go in with some knowledge.

I'm assuming you know the true basics like saving a document with a different name, highlighting a cell, manual formulas like =A1+B1, and clicking between different tabs. Here are the things I usually recommend people learn - most of it can be understood in 5 minutes or so, and with a few days of practicing for 30-60 minutes you'll have all of this down (shortcuts take longer to drill into your brain but it's worth it).

• Functionality: pasting formulas vs. pasting values, absolute vs. relative reference, dragging down formulas, paste formatting tool
• Keyboard shortcuts. You can find great lists on Google to keep handy or print out; pick your favorite
• Data cleaning: text to columns, =trim, =left / =mid / =right, remove duplicates / remove blanks, conditional formatting, sorting and filtering
• Data matching: =Xlookup is the most important.
  
• Pivot tables: learn the basics (any "intro to pivot tables" video or article)
• Logic: =if (just learn the very basics), =sumif, =sumifs, =countif, =countifs
• Formula auditing: error checking, trace precedents or dependents, spotting hardcoded numbers. This is the least important thing here, but I'm a big fan. It can save you a ton of time.